The present invention relates to electronic medical instruments and more particularly to an electroencephalograph instrument for the analysis and display of abnormal brain states.
In medicine it is frequently of vital importance to determine the extent and type of a patient's abnormal brain states. For example, when a patient is in a comatose state due to an anesthetic during surgery or other procedures, it may be important to monitor the brain state of the patient to determine if more or less anesthetic is needed, if the anesthetic should be changed, and if the procedure may continue or should be halted or altered. As another example, in hospital intensive care the brain state of a patient, at risk for brain damage for a variety of possible reasons, may be monitored to aid in diagnosis and to determine the optimal level and type of medication or other treatment. In addition, comprehensive quantitative analysis and display of a patient's abnormal brain state may be of vital importance in the differential diagnosis and evaluation of various neurological diseases, assessment and prognosis of head injury victims or discrimination between the so-called "flat EEG" of cerebral death versus the effects of deep barbiturate intoxication.
There is considerable interest and discussion regarding the medical and legal definition of death and its relationship to cerebral death. There has been a considerable amount of publicity regarding the question of whether a comatose patient may be, either for medical or legal purposes, considered or declared dead. This type of question arises in a number of different contexts. For example: Should the use of life support systems be continued for long-term patients in a deep coma? Should organ transplants from comatose patients be permitted and at what stage? and, In the event of limited medical facilities, for example, in an emergency or a battlefield situation, which comatose patients should receive treatment efforts and facilities?
The present inventor, in his prior U.S. Pat. No. 3,706,308 entitled "Life Detecting Medical Instrument", attempted to provide a portable device which would determine the presence or absence of life in a comatose patient. That instrument had certain limitations as to sensitivity; for example, it may not have been able to accurately determine whether a patient was in a deep coma due to barbiturates or had suffered cerebral death. The instrument of the present invention is devised to extend this sensitivity by addition of many additional measures of brain activity, to increase precision by providing separate quantitative indices of the state of different regions of the cortex, thalamus and brain stem, to provide a basis for quantitative prognosis of the outcome as well as differential diagnosis of brain damage or dysfunction, and to provide a multivariate brain state vector (BSV) to facilitate presentation and comprehension of information about the quality, severity and stability of abnormal brain states.
The objective and precise determination of death may be critical for optimal but ethical organ transplants, such as heart transplants. If organ transplantation is to be successful, the organ must be removed as soon as possible after death. If the removal of the heart is delayed, there is a risk that it will be damaged and unusable. The removal of a vital organ precludes any revival of the donor's life. A doctor or hospital may run the risk of civil or even criminal liability if it is later held that a patient was not legally dead at the time the vital organ was removed.
Traditionally, medical science has accepted the classical definition of death as being a total stoppage of the circulation of the blood and a cessation of the animal and vital functions, such as respiration and pulsation. Many physicians now doubt that the traditional definition of death is adequate. A more modern and accepted definition of death is based upon the cessation of brainwave activity; for example, see Hamlin, Life or Death EEG, 190 J.A.M.A., 1964. Death is defined as occurring when the spontaneous brain electrical activity, which is measurable on an electroencephalograph (EEG), is isopotential or "flat" (without brain waves two microvolts (2.mu.V) in amplitude) measured by specified amplifiers at specified gain.
Yet, the presence of a flat brain wave is not a reliable indication of the lack of life. Cases have been reported with isopotential EEG and subsequent recovery of the patient, especially after suicidal or accidental ingestion of large doses of barbiturate. Further, the presence of amplifier noise causes decisions about the presence or absence of low amplitude EEG activity to sometimes be equivocal. Even with flat EEG after barbiturate overdoses, the brain may remain electrically reactive to sensory stimulation and recovery can occur.
Neurophysiologists presently employ average response computation to enhance the signal-to-noise ratio of the electrical responses of the brain to sensory stimuli. A series of strong stimuli is delivered to the sensory receptors of the organism and the average evoked response of the brain, or "EP", is examined for transient brain wave reaction phase-locked to the stimulation. Noise is not phase-locked, so that averaging the brain wave activity for a series of stimuli provides an enhancement of signal-related potentials. Since particular sensory systems may be damaged in a given patient, preferably one should test three of the major sensory systems. Presence of a non-zero sensory EP constitutes unequivocal proof of life.